1. Field of the Invention
The present invention relates to a hot isostatic pressing apparatus and a hot isostatic pressing method.
2. Description of the Related Art and Problems to be Solved by the Invention
As a technique for eliminating residual pores in casting products or in sintered products such as ceramics by performing a heat-up under a high-pressure gas atmosphere of several tens to several hundreds MPa, hot isostatic pressing (HIP) has been extensively used in industrial fields with confirmation of effects such as improvement in mechanical characteristic, reduction in scatter of properties, and improvement in yield.
A conventional HIP apparatus used for the purpose as described above has, as shown in FIG. 7, a structure containing a so-called resistance-wire heating electric furnace, in which a bottomed cylindrical heat insulating structure 44 and a resistance-wire heater 45 arranged along the inner surface of the heat insulating structure 44 are provided within a vertically cylindrical high-pressure vessel comprising a high-pressure cylinder 41 and upper and lower lid 42 and 43. The upper lid 42 comprises a high-pressure gas inlet port 46, and the lower lid 43 is formed of a ring-like lid 47 and a plug-like lid 48 on the inside thereof. At the time of operation, a workpiece housing case 50 containing workpieces 49 is placed on the plug-like lid 48 through a workpiece base 51, and charged into a treatment chamber 52 on the inside of the resistance-wire heater 54. Since such an apparatus is operated under rather high pressure, the load of internal pressure acting on the upper and lower lids 42 and 43 reaches up to 8000 tons in the operation of such a large apparatus as to have an inside diameter of 1 m at 100 MPa. Therefore, the upper and lower lids 42 and 43 are supported by window-frame shaped steel frames (not shown) in order to support this load.
The resistance-wire heater 45 is vertically divided and arranged in two or more stages so as to surround the treatment chamber 52 in the high-pressure vessel. This is a measure to ensure the vertical temperature uniformity in the treatment chamber 52 because the violent natural convection of the high-pressure gas is apt to cause in the treatment chamber 52 a temperature distribution that the upper part has a high temperature and the lower part has a low temperature. Further, the natural convection of the gas also causes the excessive dissipation of the heat for heating the treatment chamber 52. In order to efficiently suppress it, therefore, the structure of surrounding the treatment chamber 52 and the heater 45 with the bottomed cylindrical heat insulating structure 44 is adapted. The heat transmitted to the high-pressure cylinder 41 of the high-pressure vessel through the heat insulating structure 44 is removed by the cooling water running in a water cooling jacket part 53 on the circumference thereof. Denoted at 54 and 55 are cooling water inlet port and outlet port, respectively.
The operation for temperature and pressure in the conventional HIP method as described above comprises processes of heat-up and pressurization; temperature and pressure retaining; and temperature reduction (cooling) and pressure reduction, as shown in FIG. 8, after the evacuation and gas substitution for discharging the internal air of the HIP apparatus prior to treatment. As is apparent from FIG. 8, the long overall treatment time, particularly, the long residence time of workpieces in the expensive high-pressure vessel results in an increase in treatment cost, and a reduction in the cycle time has been thus a serious subject in industrial productions. In order to improve this, particularly, the long time occupied by the cooling process, namely the slow cooling rate, research and development have been carried out with respect to a so-called rapid cooling technique for HIP method.
In a cooling method by use of natural convection, which was proposed at the beginning, the cooling rate falls as the temperature difference between the treatment chamber part and the space just inside of the inner surface of the high-pressure vessel becomes smaller. Therefore, in order to improve it, it was proposed to place a fan or a pump inside the high-pressure vessel to generate a forced convection in addition to the natural convection (e.g., a HIP apparatus proposed in Japanese Utility Model Publication No. 3-34638). However, since even the method for generating the forced convection by use of the fan or pump to improve the cooling rate adapted the type of performing a heat exchange between a circulating gas forcedly circulated in a heat insulating layer and a circulating gas forcedly circulated in a furnace chamber, a large temperature distribution with a high temperature in the upper side and a low temperature in the lower side still occurred in the inner part of the furnace chamber (treatment chamber) although an improvement in cooling rate was recognized up to a certain temperature range. Consequently, the problem that the thermal histories of workpieces in the treatment chamber are varied depending on places became to be recognized.
In an HIP apparatus structure proposed in U.S. Pat. No. 4,532,984, since the gas forcedly circulated by a pump or fan is alternately carried in the respective inside and outside spaces of a heat insulating hood and an inner cylindrical liner, and a heat exchanger comprising a coiled tube is set adjacently to the pump, the cooling rate can be enhanced more than in the HIP apparatus structure proposed in the above Japanese Utility Model Publication No. 3-34638. However, an uniform circulating gas flow cannot be obtained because the pump for forcedly circulating the gas in the inside and outside spaces of the heat insulating hood is positioned closer to one side of a lower lid. Further, uniform cooling of the circulating gas cannot be expected because the heat exchanger is also shifted in conformation to the setting position of the pump. Therefore, the problem that the thermal histories of the workpieces in the treatment chamber are varied depending on places is supposed.
By using the method for improving the cooling rate as proposed in each patent publication described above, the retaining temperature, for example, the cooling rate from 1200° C. to about 300° C. could be significantly improved. On the other hand, it is preferable to cool the workpieces to 100° C. or lower when taking out and handling the workpieces from the viewpoint of operator's safety such as prevention of a disaster by burn. However, the limitation of temperature (substantially a surface temperature of 100 to 150° C.) in the high-pressure vessel as heat sink (temperature lowering device) still remains even with the above method, and it is the actual condition that a sufficient function cannot be necessarily provided in the point of cooling to 100° C. or lower.
In recent years, further, there has been proposed the idea of performing quenching simultaneously with HIP treatment, and a super-rapid cooling and the temperature uniformity in the treatment chamber inner part is required for this. To meet the requirements, a heat sink for releasing the heat possessed by the treatment chamber including the workpieces and the heat insulating structure in a short time is required, which has developed the recognition that a sufficient heat extraction effect cannot be obtained only with the conventional thought of using the high-pressure vessel itself as the heat sink. Particularly, in such a large HIP apparatus as to have a treatment chamber diameter exceeding 1 m, such a requirement is increasingly enhanced.